1. Field of the Invention
The present invention relates to an aqueous emulsion composition, an adhesive, and a method for thickening an aqueous emulsion.
2. Discussion of the Background
An aqueous emulsion having as the dispersant, a polyvinyl alcohol polymer (hereinafter referred to as PVA polymer) has extremely high mechanical stability, chemical stability and miscibility with pigment, as the PVA polymer acts as protective colloid. In addition, films formed from these aqueous emulsions are extremely stiff. Aqueous emulsions have many applications in various fields of coating compositions, adhesives, fiber processing agents, paper processing agents, etc.
An aqueous emulsion having a PVA polymer as the dispersant is characterized by so-called shear thinning (hereinafter referred to as thixotropy), which generally indicates the decrease in the apparent viscosity of a fluid with the increase in the shear rate. In practical uses of such aqueous emulsions, shear-thinning is often preferred.
Recently, it is desirable to speed up production of products e.g., wood bonded products, paper bonded products, fiber products, etc. made from an aqueous emulsion having a PVA polymer as the dispersant. This is typically accomplished, for example, by accelerating the speed of roll rotation in the line. In this situation, the conventional, shear-thinning aqueous emulsion has problems because it is difficult to control the amount of emulsion applied to the rolls and, after it has been applied, the emulsion scatters around the rolls.
To solve these problems, an aqueous emulsion is often processed into a Newtonian one wherein the apparent viscosity does not depend on the shear rate, or into a shear-thickening one wherein the apparent viscosity increases with an increase in shear rate (the latter may be hereinafter referred to as dilatant fluid).
To convert a thixotropic, aqueous emulsion having a PVA polymer as the dispersant into a Newtonian one, for example, the emulsion polymerization method for producing the emulsion may be improved by controlling the amount of the initiator and the dispersant, or the PVA polymer may be defined with respect to the degree of hydrolysis and molecular weight.
Where such a Newtonian, aqueous emulsion is desired to be further converted into a dilatant one, it is generally difficult to convert the viscosity or fluidity of the emulsion by merely improving the emulsion polymerization method or specifically defining the PVA polymer to be used as the dispersant in the manner as above while the dispersion stability of the aqueous emulsion is still kept stable. For this, therefore, to convert the thixotropic emulsion into a dilatant one, it may be possible to employ a method of adding to an aqueous emulsion having a thixotropic PVA polymer as the dispersant, a gelling agent for the PVA polymer in the step of post-processing the emulsion. However, this method has problems because the gelling agent is not safe and would unfavorably color the emulsion. In addition, the use of the emulsion produced would be limited.
Japanese Patent Application Laid-Open (JP-A) No. Hei-6-211911 discloses an aqueous copolymer emulsion for processing paper, which is obtained by emulsion polymerization using a naphthalenesulfonate-formaldehyde condensate and a polyoxyethylene alkylphenyl ether sulfate, and is characterized by its high-speed coatability. The laid-open patent application discloses that PVA may be optionally added to the emulsion, but is silent regarding the meaning of the combination of PVA and the naphthalenesulfonate-formaldehyde condensate, the compositional ratio of the two in the emulsion, and the amount of PVA being larger than that of the condensate in the emulsion.